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Magnetic Fields Magnetic fields • Produced by two kinds of electron motion – electron spin • main contributor to magnetism • pair of electrons spinning in same direction creates a stronger magnet • pair of electrons spinning in opposite direction cancels magnetic field of the other – electron revolution © 2010 Pearson Education, Inc. Magnetic Domains Magnetic domains • Magnetized clusters of aligned magnetic atoms Permanent magnets made by • placing pieces of iron or similar magnetic materials in a strong magnetic field. • stroking material with a magnet to align the domains. © 2010 Pearson Education, Inc. Magnetic Domains Difference between permanent magnet and temporary magnet • Permanent magnet – alignment of domains remains once external magnetic field is removed • Temporary magnet – alignment of domains returns to random arrangement once external magnetic field is removed © 2010 Pearson Education, Inc. Magnetic Domains © 2010 Pearson Education, Inc. Magnetic Forces on Moving Charges Moving charges in a magnetic field experience a deflecting force. (continued) © 2010 Pearson Education, Inc. Earth’s Magnetic Field • Earth is itself a huge magnet. • The magnetic poles of Earth are widely separated from the geographic poles. • The magnetic field of Earth is not due to a giant magnet in its interior—it is due to electric currents. • Most Earth scientists think that moving charges looping around within the molten part of Earth create the magnetic field. © 2010 Pearson Education, Inc. Earth’s Magnetic Field • Universe is a shooting gallery of charged particles called cosmic rays. • Cosmic radiation is hazardous to astronauts. • Cosmic rays are deflected away from Earth by Earth’s magnetic field. • Some of them are trapped in the outer reaches of Earth’s magnetic field and make up the Van Allen radiation belts © 2010 Pearson Education, Inc. Electric Currents and Magnetic Fields Connection between electricity and magnetism • Magnetic field forms a pattern of concentric circles around a current-carrying wire. • When current reverses direction, the direction of the field lines reverse. © 2010 Pearson Education, Inc. Right Hand Rule for a current carrying wire Electric Currents and Magnetic Fields © 2010 Pearson Education, Inc. Electric Currents and Magnetic Fields Magnetic field intensity • increases as the number of loops increase in a current-carrying coil temporary magnet. © 2010 Pearson Education, Inc. Electric Currents and Magnetic Fields Electromagnet • Iron bar placed in a current-carrying coil • Most powerful—employs superconducting coils that eliminate the core • Applications – control charged-particle beams in high-energy accelerators – lift automobiles and other iron objects – levitate and propel high-speed trains © 2010 Pearson Education, Inc. Magnetic Forces on Moving Charges Moving charges in a magnetic field experience a deflecting force. ! ! FB = q!v × B • Greatest force – particle movement in direction perpendicular to the magnetic field lines • Least force – particle movement other than perpendicular to the magnetic field lines • No force – particle movement parallel to the magnetic field lines © 2010 Pearson Education, Inc. Magnetic Forces on Moving Charges Moving charges in a magnetic field experience a deflecting force. ! ! FB = q!v × B • Greatest force – particle movement in direction perpendicular to the magnetic field lines • Least force – particle movement other than perpendicular to the magnetic field lines • No force – particle movement parallel to the magnetic field lines © 2010 Pearson Education, Inc. In General: For Magnetic Force ! F!B = q!v × B !v ! B Earth’s Magnetic Field • Storms on the Sun hurl charged particles out in great fountains, many of which pass near Earth and are trapped by its magnetic field. The trapped particles follow corkscrew paths around the magnetic field lines of Earth and bounce between Earth’s magnetic poles high above the atmosphere. • Disturbances in Earth’s field often allow the ions to dip into the atmosphere, causing it to glow like a fluorescent lamp. Hence the aurora borealis or aurora australis. © 2010 Pearson Education, Inc. Storing electrical energy Consider a positive and a negative charge - + Separating them takes energy - + The separated configuration has more electrical potential energy. Which means it has more voltage associated with it. This is how batteries work: Chemical reactions separate charges. Summary: Current, Voltage, etc. • Charge q. (Positive and Negative) • Electrical potential energy: Eelec = qV. • Electrical potential (i.e. Voltage) V, Units (SI): Volts (V) • Current (I): I = q/t. Units (SI): Amperes (“Amps,” A) • Power P = I V. (SI units: Watts! (if Volts, Amps)) • Series circuits: voltages add • Parallel circuits: voltage same; currents add